1. Field of the Invention
The present invention relates to a frequency-shift keying (FSK) signal detector for detecting an FSK signal, and more particularly to such a detector for detecting an FSK signal by digital processing.
2. Description of the Background Art
As a conventional FSK signal detector that detects an FSK signal by digital processing, for example, there is one which converts the output of an analog receiver to binary information, then detects frequency components by digital signal processing, and carries out demodulation. The conventional FSK frequency detector receives an input signal converted to an intermediate frequency (IF) and makes demodulation possible. In the case where the center frequency of the input IF signal to the FSK frequency detector preserves the following relationship, this conventional demodulation is possible.
Center frequency f0 of IF≧(Frequency fd equivalent to the modulating speed)×1.25 (for minimum shift keying)fd=(f0−fs)×4=(fM−f0)×4.
In the case of minimum shift keying, two frequencies converted from a binary information signal are equal to two values shifted from the center frequency of an intermediate frequency (IF) by one-fourth of a frequency equivalent to the modulating speed, and the lower of the two frequencies has to be higher than the frequency equivalent to the modulating speed.
For instance, in the case of having correlation filters with correlation coefficient sequences C1 to C2n and D1 to D2n which have wavelengths of frequencies equivalent to a frequency fs lower than the center frequency of an intermediate frequency converted by modulation and to a frequency fM higher than the center frequency and which are the same in duration as the time length of one symbol, if the correlation coefficient sequences are longer, the bass band width of the correlation filters is narrower and therefore the property of removing noise is increased, but if they exceed the time length of one symbol, a modulated component of an adjacent symbol is added and therefore intersymbol interference takes place. For that reason, it is preferable that the correlation coefficient sequences be the same as the symbol time length. Therefore, demodulation is possible by the use of filters with correlation coefficient sequences which has the same length as the symbol time length and the same period as the frequency of fs or fM.
To simplify a circuit configuration that receives an FSK signal, Japanese Patent Laid-Open Publication No. 23248/1997 discloses a circuit configuration which converts the output of an analog receiver to binary information, then detects frequency components by digital signal processing, and performs demodulation. U.S. patent application Publication No. 2003/0004664 A1 to Ward et al., discloses a method of measuring jitter in serial data having periodic patterns.
However, if the frequency equivalent to the modulating speed always has the relationship of(f0−fs)×4=fd f0=fs+(fd/4),then the following expressions will be obtained:f0<fd×1.25fs+(fd/4)<fd×1.25fs<fd.Therefore, the wavelength of the frequency (fs) modulated to a lower frequency side than the center frequency is longer than the symbol time length. For that reason, in correlation filters, there is a problem that the wavelength of a modulated frequency cannot be arranged within the time length of one symbol. On the other hand, if the correlation coefficient sequences of correlation filters are made longer to obtain a correlation at a period of time longer than one symbol period, then the problem of the degradation of receiver characteristics will arise.
Thus, it is found that in the case of (center frequency of IF)<(frequency equivalent to the modulating speed)×1.25 (for minimum shift keying), the conventional method cannot be utilized.